Window member for an x-ray device

ABSTRACT

A window member for separating an internal environment of an x-ray device from an environment external to the x-ray device is provided. The window member comprises a substrate and a coating layer disposed upon a surface of the substrate. The substrate is formed from a polycrystalline material and is substantially transparent to low-energy x-rays. The coating layer is non-porous, covers the crystal grains at the surface of the substrate and extends into the grain boundaries therebetween, such that the coating layer forms an impermeable barrier between the substrate and the external environment.

FIELD OF THE INVENTION

The present invention relates to a window member for an x-ray device,and in particular for separating an internal environment of an x-raydevice from an environment external to the x-ray device.

BACKGROUND TO THE INVENTION

X-ray photons generated or to be detected in sensors are typicallyrequired to be transmitted through windows that separate ambient airfrom the vacuum environments typically found in x-ray devices. X-raydevices have radiation exit or entrance windows that sustain thedifference in environments between the inside and the outside of thedevice while allowing x-rays to be transmitted through the window. Anx-ray window typically comprises a window member generally formed from asheet of material that is placed over an opening or entrance to thex-ray tube or x-ray detector, or other x-ray device. Low atomic numbermaterials are generally used to form these windows as they allow x-raytransmission in the low energy region corresponding to 0.5-10.0 keV.

X-ray windows are typically made from a wide range of materials of lowatomic number, including beryllium, aluminium, titanium, siliconnitride, and diamond. These window materials suffer from two deleteriouseffects owing to their exposure to ambient environments which cause thewindows to fail. Firstly, since x-ray windows are typically formed frompolycrystalline materials, gas may penetrate through the boundariesbetween crystal grains. In addition to this, the materials suffer fromcorrosion caused by the material reacting with ambient water vapour,leading to the formation of an oxide layer on the window surface. Suchoxide layers, once formed, may then peel off, thus gradually reducingthe thickness of the windows over time. Eventually, the thickness of anx-ray window may be reduced to such a thickness that it can fail tosustain a vacuum inside the x-ray device.

Conventionally the solution to the problem of window failure involvescoating the window with a protective layer that is resistant tocorrosion. Candidate materials for this are required to have minimalsoft x-ray attenuation qualities, which is determined by the massattenuation coefficient and the thickness of the coating material. Forthis reason, coatings that are as thin as possible and have constituentelements with low atomic numbers are most suited to this purpose.Additionally, such windows should be capable of withstanding pressuresthat result from differential pressure between internal and externalenvironments, gravity, and thermal stresses.

Efforts have been made in the past to overcome the problem of corrosionof x-ray windows. Several approaches have aimed to solve thisproblem—for example U.S. Pat. No. 5,226,067 and U.S. Pat. No. 5,740,228.U.S. Pat. No. 5,226,067 introduced coating with amorphous boron hydride(a-B:H) and other amorphous boron hydride alloys (a-B:X:H) where X isanother element of low atomic number. However, these layers have poorthermal stability, decomposing at 200° C., which precludes standardbrazing technology that requires processing temperatures as high as 700°C. The other disadvantage of boron hydride-based coatings is that growthchamber during thin film growth is contaminated with theboron-containing gas and so is not available for other processes.Therefore a separate device is required for the B:H:X coating process.U.S. Pat. No. 5,740,228 used silicon oxide (SiO₂), silicon nitride(SiN), silicon carbide (SiC), and amorphous carbon (a-C) as coatinglayers. These coating layers were grown using sputtering and chemicalvapour deposition (CVD) methods and the thickness varied between 300 nmand 500 nm. However, sputtering growth is a non-equilibrium, highlyanisotropic growth method which means that grain boundaries form,providing channels for air to contact the window and to begin thedegradation process. With CVD the quality of the layer is better.However, the high growth rate of this method makes it difficult tocontrol film thickness to nanometre scale.

In view of this, a need exists for an x-ray window that is not prone todeterioration caused by exposure to external environments and is alsooptimally transparent to low energy x-rays. The present invention isaimed at solving this problem.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a window member forseparating an internal environment of an x-ray device from anenvironment external to the x-ray device, the window member comprising asubstrate and a coating layer disposed upon a surface of the substrate,wherein the substrate is formed from a polycrystalline material and issubstantially transparent to low energy x-rays, and the coating layer isnon-porous, covers the crystal grains at the surface of the substrate,and extends into the grain boundaries therebetween, such that theircoating layer forms an impermeable barrier between the substrate and theexternal environment.

The inventors have realised that the deterioration and failure of x-raywindow members can be alleviated by providing a substrate in which boththe penetration of gas through grain boundaries and the oxidation of thesubstrate material, such as beryllium, is prevented or reduced by thepresence of a coating layer that isolates and protects the substratefrom the external environment. The window member advantageously includesa coating layer that is impenetrable by environmental factors such asair or oxygen and moisture, and covers and conforms to the externalsurfaces, including those that extend into the grain boundaries, ofcrystal grains in the substrate that would, in the absence of thecoating layer, be exposed to such factors.

In the window members of the prior art, air molecules taking pathsbetween grain boundaries by way of diffusion processes could causeleakage and eventual device failure. Coating of x-ray window substrateswith materials that are resistant to corrosion can significantly reducethe corrosion suffered by the window substrate since grain surfaces arecovered with deposited materials, and no holes and no pores are formedwithin the coating layer through which the window member may be exposedto ambient air.

The penetration of the coating layer into the boundaries between thecrystal grains at the surface of the substrate means that the barrierdefined by the coating extends continuously over, and covers all of, thesurface of the substrate so that no part of the surface may be contactedby external environment atmosphere, gas, or fluid. The extent to whichthe coating layer extends or penetrates into the grain boundaries istypically defined by the size or width of the grain boundary at aparticular depth, that is by the scale of the gap between adjacentgrains. The coating layer typically extends as deep into a given grainboundary as the depth below the outermost surface of the grains at whichthe size of the gap reaches the order of atomic size. Typically, and inmaterials from which x-ray window substrates are commonly constructedsuch as beryllium, the width of a grain boundary gap narrows withincreasing depth below the outer surface, such that at a depth of around100 nm below the surface of the substrate or of the outer crystalgrains, the distance between the grains is at the atomic scale.Therefore the coating layer of the window member typically extends intothe grain boundaries to a depth of at least 100 nm below the surface. Inother words, the coating layer typically extends into each of the grainboundaries of the window member to a depth at which the space in betweenthe grains at the boundary is on the atomic scale. At this depth, thegaps between grains are too small for atomic layer deposition (ALD)precursors to diffuse so as to coat grain boundaries during coatinglayer growth.

The depth to which the coating layer extends into the grain boundariesmay typically be measured or defined as the depth below or into theoutermost extremity of the rough, polycrystalline surface, the averageplane or height of the crystal surface. The depth is typically definedin a “vertical” direction, that is, normal to the plane of the substrateor the substrate surface. The depth within a boundary may be defined asthe “vertical” distance between the top of a grain and the first jointpoint with a neighbouring grain, that is where it contacts or is anatomic-scale distance from it, with which the given grain shares theboundary. This depth is influenced by the roughness or finish of theberyllium disc surface, which may vary between manufacturers oraccording to surface requirements.

As noted above, it is advantageous that there are no gaps in the coatinglayer through which a portion of the substrate surface might be exposedto an external environment. For this reason, a coating layer typicallyforms a continuous film having a uniform thickness and covering thesurface of a substrate.

The substrate, as noted above, is transparent, or substantiallytransparent, to low-energy x-rays. In order for the attenuation ofx-rays attributable to the coating layer to be minimised, the thicknessof the coating layer is typically less than 200 nm. In order to providereliable coverage and protection against the external environment, thecoating layer on the substrate is typically 10 nm thick or above.Depending upon the constituent materials selected for the coating layer,the thickness of the coating layer may range, in some embodiments,between 10 nm and 5000 nm, also depending upon the coating method.

A coating layer is typically of uniform thickness across the entiresurface of the substrate. The coating layer typically conforms to thesurface profile of the substrate as defined by the crystal grains andgrain boundaries. In other words, the topography of the coating layer issuch that the surface of the coating layer follows the profile of thesubstrate surface and the features thereon. Thus each crystal grain atthe surface of the substrate typically has its exposed portion entirelycovered by a portion of the coating layer, such that each adjacentportion of coating layer covering respective adjacent crystal grains atthe surface of the substrate meet each other along the edges in thegrain boundaries where the widths of the boundaries are at the atomicscale.

The porosity of the coating layer is typically less than or equal to 1%.Such a degree of porosity provides an advantageous degree of protectionfrom gases and liquids so that these fluids may not reach and affect thewindow substrate.

Typically, the pinhole density of the coating layer is less than 10cm⁻². This is a sufficiently low density of pinholes in the surface ofthe coating layer to prevent significant corrosion or oxidation of thesubstrate surface.

As noted above, the attenuation of the coating layer is important to theperformance of the x-ray window member. Attenuation is quantified as adecrease of x-ray transmission for a substrate coated with a coatinglayer as a fraction of the transmission through an uncoated substrate,generally at the x-ray energy at which the degree of transmissionthrough the uncoated substrate is 50%. FIG. 8 demonstrates thisdecrease, plotted alongside the transmission for the uncoated substrateX at a photon energy of around 2,500 keV. FIG. 8 shows the dependence ofx-ray transmission upon incident photon energy, for both a coatedsubstrate and an uncoated substrate. Typically, it is desired that thedecrease Δ is less than 5% of X. Typically, therefore, the attenuationof low energy x-rays caused by the coating layer is less than or equalto 5% of the attenuation of low energy x-rays caused by the substrate.

In order to achieve a window member that does not significantly affectthe transmission of x-rays therethrough, the thickness and theconstituent material of the coating layer are typically selected incombination such that the attenuation of low energy x-rays caused by thecoating layer is less than or equal to 5% of the attenuation oflow-energy x-rays caused by the substrate. Preferably, the coating layeris formed from or, more preferably, only from materials comprisingelements having atomic number between 5 and 80. Lower atomic numberconstituent materials give rise to lower levels of x-ray attenuation,and so the above-recited range of atomic numbers may be applied.

In some embodiments, the coating layer comprises an adhesion layer and aprotection layer, wherein the adhesion layer affixes the protectionlayer to the surface of the substrate, and wherein the protection layeris non-porous. Typically, the first layer to be applied to a substratesurface, in such embodiments, is used to achieve good adhesion to thesubstrate. The second layer to be applied thereupon is typicallyselected and used for protecting the substrate from corrosion.Multi-layer window members may provide coating layers with a lowerpinhole density in comparison with double layer coating layers.

Preferably, the porosity of the protection layer is sufficiently low toprevent the surface of the substrate being exposed to atmospheric gas orliquid.

In some embodiments, the adhesion layer comprises Al₂O₃, and theprotection layer comprises TiO₂. Either of these sub-layer materials mayalso be used in combination with any other suitable materials for eitherthe adhesion or coating layers. Thus, such a two-layer coating is notlimited to the combination of Al₂O₃ and TiO₂), but may comprise othercombinations with the same functionality.

In other embodiments, coating materials may include single layers oftitanium oxide, aluminium oxide, zirconium oxide, tungsten oxide,titanium nitride, and multiple-layer aluminium oxide, zirconium oxide.Other examples of coating materials include titanium carbide, titaniumdiboride, boron nitride, boron carbide, and their combinations (TiNO,TiBN, TiAlSnN, TiAlN, etc.).

The thickness of each of the adhesion layer and the protection layer istypically less than 200 nm. This is advantageous in that it minimisesthe attenuation caused to x-rays transmitted through the window member.

Typically, the coating layer is disposed on the substrate by way ofatomic layer deposition. This process provides the conformal andpenetrating layer growth that results in a coating layer of even anduniform thickness and applied evenly over the surfaces of the grains andinto the grain boundaries. This is advantageous for the protectivepenetrative quality of the coating layer. In some other embodiments,chemical vapour deposition is used to apply the coating layer to thesurface of the substrate.

In some other embodiments, other traditional thin film growth techniquesmay be used, such as DC/RF sputtering and pulsed laser deposition (PLD).Most preferably, atomic layer deposition is used to apply the coatinglayer, since this technique is superior to the sputtering and CVDmethods in terms of the quality of the films produced and the precisionwith which film thickness can be controlled. X-ray windows may be coatedeither by coating a bare substrate prior to brazing the substrate into awindow assembly, or by coating the window assembly after brazing.

In some embodiments, the coating layer comprises an electricallyconductive material. Preferably, the coating layer has a conductivitygreater than 10⁻⁶ Sm⁻¹. In order to achieve the desired conductivity,the coating layer may comprise titanium dioxide doped with niobium, forexample.

It is beneficial if transparency in the low energy x-ray band ismaximised in the window member. Preferably, therefore, the substrate issubstantially transparent to x-rays in the 0.5-10.0 keV energy band. Thesubstrate preferably has a transmissivity greater than 90% for lowenergy x-rays, or for x-rays in the abovementioned energy band.

Transparent or substantially transparent substrates are advantageouslyused in combination with coating layers that likewise do notsignificantly attenuate x-ray transmission in the relevant energy band.Typically, the transmissivity of the substrate and the coating layer incombination is greater than 85% for x-rays in the 0.5-10.0 keV energyband.

Preferably, the deleterious effect of the coating layer on the effectivetransparencies to x-rays of the window member is minimal, and preferablyis within 15% or more preferably 5%. Preferably, the transmissivity ofthe substrate and the coating layer in combination is greater than 95%of the transmissivity of the substrate alone.

In some embodiments, the substrate is formed from beryllium. In otherembodiments, the substrate may be formed from aluminium, titanium,diamond, or silicon nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

Some examples of the methods and product according to the invention arenow described, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of four stages of an atomic layerdeposition process;

FIG. 2 shows a top view of a window member according to the inventionalongside a perspective view of an assembly of an x-ray devicecomprising a window member according to the invention;

FIG. 3 schematically shows a portion of the surface of a window memberaccording to the invention;

FIG. 4 shows the dependents of coating layer thickness upon the atomicnumber of coating layer constituent elements;

FIG. 5 shows transmission spectra for various substrates and coatinglayers according to the invention;

FIG. 6 schematically shows an uncoated substrate alongside a coatedsubstrate according to the invention, both before and after exposure tosimulated high humidity conditions;

FIG. 7 shows output x-ray spectra for x-ray tubes with uncoated windowstogether with that of a tube comprising a coated window member accordingto the invention; and,

FIG. 8 shows the dependence of transmissivity of x-ray windows on photonenergy.

DESCRIPTION OF EMBODIMENTS

With reference to, FIG. 1, four stages of an example method forproducing a window member according to the invention are shown.Specifically, the illustrated example is an atomic layer deposition(ALD) process. This is an ultra-thin coating method based on cyclicdeposition and reaction. During such coating processes, two or morechemical vapours or gaseous precursors react sequentially on a substratesurface, producing a solid thin film. In such methods, an inert carriergas flows through a system and precursors are injected by way of veryshort pulses into the carrier flow that transports the precursor into areaction chamber.

At stage A a surface of a substrate 103 is exposed to a suitable gaseousreactant 121 which, upon reacting with the surface 103, forms amonolayer.

At stage B, excess reactant 121 and any by-products are purged out, orevacuated, by way of passing inert gases such as argon or nitrogen intothe chamber, thus displacing the reactants or by-products.

At stage C (for an oxide film) an oxidant 123 such as atomic oxygen (O),hydrogen peroxide (H₂O₂), ozone (O₃), or water (H₂O), is introduced as apulse into the chamber or tube. This has the effect of fully oxidisingthe previously adsorbed layer 122, thereby forming an oxide. Otherreactants are used to form nitrides or carbides, as is well known in theart.

At stage D the excess oxidant 123 is purged out of the chamber or tube,again by way of delivering a pulse of argon or nitrogen or anothersuitable inert gas. Thus the substrate 103 coated with an oxide coatinglayer 106 is formed. Depending upon the desired thickness of the coatinglayer 106, the cycle of deposition illustrated through stages A-D may berepeated accordingly.

The result of the ALD process is a highly homogeneous and conformal filmthat extends continuously over the entire surface of the substrate 103.Coating 106 applied using this technique extends deep into trenches inthe surface of the substrate, although no such trenches are shown in theportion of the substrate 103 shown in FIG. 1. As noted above, the numberof times the reaction is repeated determines the thickness of thedeposited coating 106.

An example window member according to the invention is shown in FIG. 2together with a portion of an x-ray device 210 in which the member isinstalled. The window member 201 comprises a polycrystalline berylliumsubstrate, wherein the micrometre sized beryllium grains are coated withan impermeable coating at a surface of the substrate. The x-ray window201 is a circular disc with a diameter that is between 5 and 20 mm and athickness that is between 12 and 500 μm.

There are a number of criteria by which the material and thin filmgrowth method of the coating layer may be selected in order for thewindow member to meet the requirements of an x-ray window.

Firstly, the coating should be uniform and should cover the entirewindow surface, or the entire surface of the substrate that is to beexposed to the environment external to the x-ray device 210. The coatingis grown on the substrate surface such that the profile of the coatingrepeats the surface profile of the substrate. A cross section of a grainboundary 308 between two adjacent beryllium crystal grains 307 in thesurface of a substrate is shown schematically in FIG. 3. The coatinglayer 306 has been applied by way of atomic layer deposition and, assuch, conforms to the surface profile of the grains 307. The coatinglayer therefore extends into the grain boundary 208 to a depth ofapproximately 100 nm below the outward most parts of the grains 207 thatdefine the plane of the substrate surface. This is due to the ALDprocess uniformly coating all parts of the exposed grain surfaces downto a depth where the gap between adjacent beryllium crystalline reachesthe atomic scale. Depths of over 100 nm are typically the point withinsurface grain boundaries at which the grain boundary gap in berylliumsubstrates narrows to this size. The isotropic growth caused by ALDtherefore leaves no part of the substrate exposed, that is, not coveredby the coating layer. In this way, the substrate of an installed windowmember 201 such as that in device 210 is fully isolated from the ambientatmosphere that is external to the device 210.

Another criterion for the coating layer is that it is in the form of alow-porosity film. That is, the coating contains no or negligibleinterstices through which any liquid or gas may pass. Thus, the coatingserves to isolate the substrate upon which the film is disposedcompletely from the atmosphere, environment, or fluids on the other sideof the coating layer to the substrate.

A further criterion is that the coating layer has a high transmissivity,of greater than 90%, to low energy x-ray radiation, that is in the0.5-10.0 keV energy band.

A further consideration is that the coating layer has a high chemicalstability, meaning that it is not reactive to the substrate or to theatmosphere or environment to which it may be exposed. Such a chemicallystable coating layer will therefore retain its impermeable structure andtotal coverage of the window so as to continue to perform as a barrierthat isolates the substrate from the external atmosphere.

The uniformity and the porosity of the coating layer depend in part uponthe growth methods used to apply it. Table 1 below lists some commonlyused growth methods (sputtering, pulsed laser deposition (PLD), chemicalvapour deposition (CVD), atomic layer deposition (ALD) and sol-gel(solution-based)). A measured porosity and three-dimensional uniformitythat is achievable using each method is set out in Table 1. It can beseen that a good degree of impermeability can be achieved using allmethods except for sol-gel. However, as can be seen from the table,isotropic layer growth, which results in the formed layer conforming tothe surface profile of the substrate and the grains therein and isindicated by 3D uniformity, can be achieved only by way of ALD and CVD.A reason for this is that isotropic growth should take place with asurface-limited reaction′, and so sputtering and PLD-based growth isdirectional and occurs predominately or solely on surfaces or regions ofthe grains which are angled towards or facing the precursor material.Since beryllium and other polycrystalline materials comprise grains attheir surface, which results in a non-smooth surface wherein the normalfactor varies according to the grain structure intersected by grainboundaries, the grain boundaries may remain uncoated when using suchmethods. In these cases, the boundaries can provide leakage paths bywhich the external environment may contact the substrate.

A further consideration for the method by which the coating layer isformed is the ability to precisely control the thickness of the coatinglayer so that a side thickness may be achieved. This is advantageoussince any excess thickness will reduce x-ray transmissivity owing to thepresence of any greater depth of coating material through which x-raysmust pass. Atomic layer deposition allows the greatest degree of layerthickness control, since it involves depositing material one atomic percycle. Various techniques for performing CVD are available, varying inthe pressure of the gas environment present during growth. Mostcommonly, CVD is performed at relatively high pressures ranging fromatmospheric pressure to the order of several mTorr. In a variation onthe previously described example method, the coating layer is appliedusing CVD at low pressure, approximately 10⁻³ Torr. This method allows afine level of control of the coating layer thickness. The CVD method ofapplying the coating layer to the polycrystalline window is suitable forexamples using very low atomic number materials to form the coatinglayer. Examples of such materials are diamond-like carbon, aluminiumnitride, and boron carbide. Owing to their low atomic number, thecoating layers produced from such materials give rise to x-rayattenuation in the desired energy band of 5% and under, in spite of thehigh growth rate and thick layers that typically result from CVD-basedcoating processes.

TABLE 1 Impermeability 3D uniformity Atomic layer deposition goodexcellent Chemical vapor deposition good good Sputtering (DC/RF) goodpoor Pulsed laser deposition good poor Sol-gel poor good

As indicated previously, the transmissivity of a coating layer to lowenergy x-rays is influenced by a combination of the atomic number (Z) ofconstituent elements of the coating layer, and the thickness of thelayer. The lower the atomic number, the greater the thickness of thelayer that can be employed in order to produce a coating of a givendesired transmissivity. Experimental data demonstrating thisrelationship is shown in FIG. 4. Here the Y axis corresponds to thethickness of the coating layer. The of x-rays after passing through theberyllium substrate and coating layer should preferably not be less than90% of the incident x-ray beam. The graph therefore illustrates thedependence of thickness of the coating layer upon the atomic number ofthe coating layer constituents that can still give rise to a coatedwindow having an overall transmissivity above a 90% threshold. It can beseen that as the atomic number of the coating layer materials increases,the maximum thickness of coating layer which results in a transmissivitygreater than a particular threshold value presenting a desired level oftransparency decreases.

In the present example, the coating layer is formed from tungsten oxide(WO₃). The atomic number for tungsten is Z=74. It has been found thatthis example coating layer is effectively transparent to low energyx-rays in the aforementioned energy band for film thicknesses up to 100nm. Measured transmissivity data for this example is shown in FIG. 5D.It has been found that coating thicknesses of 100 nm are sufficient toprovide a degree of protective environmental isolation for a substrateto produce a five-fold increase in the useful lifetime of a windowmember.

FIGS. 5A, 5B, and 5C show the measured transmission spectra for 1 μmthick Al₂O₃, 500 nm thick, ZrO₂, and 300 nm thick SnO₃ coating layersrespectively. Each of these is plotted alongside the transmissionspectrum of a 127 μm thick beryllium substrate window, in combinationwith that of a beryllium substrate 127 μm thick, forming a coated windowmember. From these comparisons it can be seen that the transmissionthrough the coating layers is comparable to the transmission through theberyllium window. Thus it can be seen that the additional x-rayattenuation attributable to the coating layers is insignificant.

A wide range of materials have been identified as being suitable, inaccordance with the criteria set out above, for being used ascorrosion-protective coating layers. Table 2 below lists a number ofexample coating layer materials alongside the growth methods of thosementioned previously which may be used to apply the respective coatinglayer materials to a substrate.

TABLE 2 Materials Growth method TiO₂ ALD, CVD Al₂O₃ ALD, CVD ZrO₂ ALD,CVD SiN ALD SiO₂ ALD TiB₂ CVD SnO₂ ALD, CVD SnO₂:Sb ALD, CVD AlN ALD,CVD GaN ALD, CVD InN ALD, CVD TiN ALD, CVD WO₃ ALD, CVD HfO₂, ALD, CVDIn₂O₃ ALD, CVD Ga₂O₃ ALD, CVD Ta₂O₅ ALD, CVD SrTiO₃ ALD, CVD CeO ALD,CVD BaTiO₃ ALD, CVD ZnO ALD, CVD ZnO:Al ALD, CVD MgO ALD, CVDBa_(X)(Y_(1−X))ZrO₃ ALD, CVD LaCoO₃ ALD, CVD Nb₂O₅ ALD, CVD NiO ALD, CVDY₂O₃ ALD, CVD LaNiO₃ ALD, CVD La₂O₃ ALD, CVD YB₂Cu₃O_(7−X) ALD, CVD CaOALD, CVD CuO ALD, CVD SiC ALD, CVD TiC ALD, CVD BC ALD Ti ALD Zr ALD NiALD

Comparative studies of beryllium substrates according to the firstexample window member, for the purpose of illustrating the resistance ofthe coating layer to corrosion, have been performed with and without acoating layer. Coated and uncoated beryllium substrates were exposed tohigh levels of ambient humidity. High humidity was simulated by way offorming water drops on the centres of the beryllium substrates, and thesubstrates were placed on a hot plate. The experiments showed thatuncoated beryllium substrates eroded over time, and formed a pit, whilethe surface of a beryllium substrate with a coating layer remainedunchanged after being contacted with moisture. These results areillustrated in FIG. 6. The schematic drawing of Figure A shows anuncoated beryllium substrate 603A with a thickness of 127 μm. Afterbeing contacted with moisture in the conditions described above, a pit619 is formed in the substrate 603A owing to the corrosion caused by thesimulated high humidity. A similar substrate 603B, to which a 100 nmthick coating layer 606 was applied formed no pit in the sameconditions. The unchanged state of the window member 601 comprising thecoated substrate can be seen in the before and after drawings at B.

Thus it was found that the rate of erosion of the beryllium substrate isdecreased significantly by the presence of the coating layer. No clearsigns of erosion at all were found in the coated beryllium window.

Additionally, experiments with brazed window assemblies were performed.In these experiments it was found that assemblies with coated substratesexhibited more than five times the useful longevity of brazed windowassemblies with uncoated windows.

Two methods of coating a substrate with corrosion resistive layers arecoating a brazed window assembly, or brazing a coated window disc. Thepreferred method will depend upon the properties of the chosen coatingmaterials. Materials capable of withstanding brazing temperatures ofaround 800° C. may be coated on their window discs, that ispolycrystalline substrate discs, before brazing. Examples of coatingsthat are suitable to be applied in this way include boron nitride, boroncarbide, and titanium nitride, which have melting temperatures greaterthan 2,500° C.

Materials with lower melting points are more suitable for beingdeposited upon window assemblies after they have been brazed. Examplesof such materials include titanium dioxide, aluminium(iii) oxide, andzirconium dioxide.

Simulations and experimental studies have been performed in order toestablish optimal or suitable coating layer thicknesses for windowmembers. These have shown that materials with constituent atomic numbersup to Z=80 can be used. The optimum thickness depends upon the massattenuation coefficient of the materials. This coefficient depends onthe atomic number and the density of the material. The greater thevalues of these parameters, the smaller the coating layer thickness mustbe in order to achieve a sufficiently high transmissivity. Bothelemental and compound materials may be used to form the coating layer.

In a second example window member, the coating layer comprises multiplesub-layers. In particular, the present example includes, as part of thecoating layer, an adhesion layer and a protection layer. These differentsub-layers, with different chemical compositions, are deposited on topof one another so as to form a stack upon the substrate. Thisarrangement comprises Al₂O₃ formed upon the substrate surface as anadhesion layer, with TiO₂ grown on top of the adhesion layer, as aprotection layer.

As noted above, the maximum allowable thickness of a coating layervaries according to the mass attenuation coefficient of the coatinglayer. The criterion for this maximum allowable thickness may bedetermined by the requirement that the spectrum of x-ray transmissionthrough the coating layer in the relevant x-ray energy band should becomparable to the transmission spectrum of the beryllium substrateitself. The spectra are plotted, as mentioned above, in FIGS. 5A-5D.

Variations of the example window members described thus far includesubstrates made from aluminium, titanium, diamond, and silicon nitride.Substrates formed from these various materials will have differentdegrees of surface roughness. Depending upon the surface roughness ofeach substrate, the minimum thickness for the coating layer that allowsthe coating layer to act as a protective, impermeable barrier, may bereduced to as little as 10 nm.

A further consideration in selecting constituent materials for a windowmember is that, as well as not significantly reducing the transmittedx-ray intensity, the window member materials must not contaminate outputx-ray spectra by way of x-ray fluorescence caused by elements present inthe window member itself. Experimental results from tests performedusing an x-ray tube comprising an x-ray window coated with a TiO₂coating layer are shown in FIG. 7. This data compares the x-ray spectraof tubes with coated and uncoated windows. It can be seen that nofluorescence lines from the coating material are present in thespectrum.

Emission peaks at 2.7 keV, 20.21 keV and 22.72 keV correspond to rhodiumL_(α), K_(α), and K_(β) lines, respectively. No titanium fluorescencelines being detected in the spectrum means that this is a suitablematerial for transmitting uncontaminated x-rays through the windowmember coated with this protective coating.

1. A window member for separating an internal environment of an x-raydevice from an environment external to the x-ray device, the windowmember comprising a substrate and a coating layer disposed upon asurface of the substrate, wherein: the substrate is formed from apolycrystalline material and is substantially transparent to low-energyx-rays; and the coating layer is non-porous, covers the crystal grainsat the surface of the substrate and extends into the grain boundariestherebetween, such that the coating layer forms an impermeable barrierbetween the substrate and the external environment.
 2. A window memberaccording to claim 1, wherein the coating layer extends into the grainboundaries to a depth of at least 100 nm below the surface.
 3. A windowmember according to claim 1, wherein the coating layer extends into eachgrain of the boundaries to a depth at which the spacing between thegrains at the boundary is the atomic scale.
 4. A window member accordingto claim 1, wherein the coating layer forms a continuous film having auniform thickness and covering the surface of the substrate.
 5. A windowmember according to claim 1, wherein the thickness of the coating layeris less than 200 nm.
 6. A window member according to claim 1, whereinthe coating layer conforms to the surface profile of the substrate asdefined by the crystal grains and grain boundaries.
 7. A window memberaccording to claim 1, wherein the porosity of the coating layer is lessthan or equal to 1%.
 8. A window member according to claim 1, whereinthe pinhole density of the coating layer is less than 10 cm⁻¹.
 9. Awindow member according to claim 1, wherein the attenuation oflow-energy x-rays caused by the coating layer is less than or equal to5% of the attenuation of low-energy x-rays caused by the substrate. 10.A window member according to claim 1, wherein the thickness and theconstituent material of the coating layer are selected in combinationsuch that the attenuation of low-energy x-rays caused by the coatinglayer is less than or equal to 5% of the attenuation of low-energyx-rays caused by the substrate.
 11. A window member according to claim1, wherein the coating layer is formed only from materials comprisingelements having atomic number between 8 and
 80. 12. A window memberaccording to claim 1, wherein the coating layer comprises an adhesionlayer and a protection layer, wherein the adhesion layer affixes theprotection layer to the surface of the substrate, and wherein theprotection layer is non-porous.
 13. A window member according to claim12, wherein the porosity of the protection layer is sufficiently low toprevent the surface of the substrate being exposed to atmospheric gas orliquid.
 14. A window member according to claim 12, wherein the adhesionlayer comprises Al₂O₃, and the protection layer comprises TiO₂.
 15. Awindow member according to claim 12, wherein the thickness of each ofthe adhesion layer and the protection layer is less than 200 nm.
 16. Awindow member according to claim 1, wherein the coating layer isdisposed on the substrate by way of atomic layer deposition.
 17. Awindow member according to claim 1, wherein the coating layer comprisesan electrically conductive material.
 18. A window member according toclaim 1, wherein the coating layer comprises TiO₂ doped with Nb.
 19. Awindow member according to claim 1, wherein the coating layer has aconductivity greater than 10⁻⁶ Sm⁻¹.
 20. A window member according toclaim 1, wherein the substrate is substantially transparent to x-rays inthe 0.5-10.0 keV energy band.
 21. A window member according to claim 1,wherein the substrate has a transmissivity greater than 90% for x-raysin the 0.5-10.0 keV energy band.
 22. A window member according to claim1, wherein the transmissivity of the substrate and the coating layer incombination is greater than 85% for x-rays in the 0.5-10.0 keV energyband.
 23. A window member according to claim 1, wherein thetransmittance of the substrate and the coating layer in combination isgreater than 95% of the transmissivity of the substrate alone.
 24. Awindow member according to claim 1, wherein the substrate is formed fromberyllium.